The present invention relates to an improved agrichemical system and method and, more particularly, to a system capable of sensing in real time the chemical condition of the soil and/or certain non-chemical parameters of interest such as organic matter, soil type and the like. This information may be utilized in real time to apply an appropriate amount of corrective agrichemical in response to a sensed deficit or excess or in response to the sensed non-chemical parameter while the apparatus is still traversing the soil sampled, or the information may be stored for later use or telemetered to a remote location. The system has important benefits in cost reduction, energy resource conservation, crop production, and reduction of environmental degradation.
Prior to the invention of U.S. Pat. No. 5,033,397 to Colburn, the modern farm practice of applying chemicals to the soil to obtain optimal crop yield differed little from that used a hundred years ago, when manure from farm animals and so-called xe2x80x9cgreen manurexe2x80x9d (composed of luguminous crops or harvest detritus) were added. The farmer, as always, desires sufficient soil fertility to ensure that a successful harvest will result from his planting. The methods by which the farmer""s objectives are met have advanced considerably. Cropland productivity is increased many-fold with the application of specific chemical materials tailored to precisely provide the plant nourishment or protection needed. Beyond the need for adequate fertility, the crop is usually also given protection from competing weeds and insects by the application of assorted herbicides and insecticides. The recommended rates of application for many of these chemicals often vary as organic matter and soil type vary.
Fertilizers and agricultural chemicals are applied by diverse types of field equipment, including granular spreaders, liquid spray bars, and anhydrous solution, or granular injectors. Farmers also make choices as to when to apply the fertilizer for the next growing season, such as in the late fall or early spring, while planting, or after planting. Similarly, agricultural chemicals such as herbicides are applied at an appropriate stage of weed growth most likely to destroy or regulate undesirable plant growth.
Assorted variables influence the amount of nitrogen and other nutrients that are available for plant growth and development. In the case of nitrogen, local field conditions determine the quantity of ammonium held on the exchange complex of the soil and the precise mechanics of conversion to more available forms via bacterial action. Conversion of variable ammonium levels at distributed oxidation levels in soils is highly variable from point-to-point even within fields which appear relatively homogeneous. Although this extreme variability of soil chemical levels has been known since at least the 1920""s, until the previous Colburn invention no one has perfected a method of accounting for this variability while adding fertilizers or other corrective chemicals such as lime.
As taught in Colburn U.S. Pat. No. 5,033,397, nitrogen exists in the soil in a variety of chemical forms. In the ammonium form it is relatively immobile, but after transformation by soil bacteria to nitrate its mobility increases drastically. Nitrate becomes elusive because of its high solubility in soil water. Nitrate moves with the soil water in response to soil temperature changes, rainfall, and crop transpiration demands. The coefficient of variation of soil nitrate levels typically has a mean of 50% and often reaches 100% even over small areas of only several square yards. Similar observations have been made for pH and potassium levels. Because available nitrogen varies widely, even when fields have been uniformly fertilized, sporadic, conventional soil samples cannot be representative indicators of a field""s nitrogen availability status.
Insufficient nutrient or herbicide levels will affect crop productivity adversely; excess levels may have a similar effect, may carry over to affect the next crop or may simply be wasted. In a field of varying organic levels and soil types, the manufacturer""s recommended rate of herbicide application would also vary but the farmer would typically be unable to vary his actual application rate in response to such variations. Farmers encountering such situations would have to utilize a constant application rate, and would typically select the highest of the various recommended rates and apply that rate of agrichemical uniformly across the field, with the aforementioned consequences. In the case of nitrogen, soil nitrate (NO3xe2x80x94N) levels above 40 ppm are considered to be wasted nutrients. Field data indicate that considerable excess nitrate is often available that does not contribute to crop production. Because nitrate is mobile and does move downward away from the rooting zone in the absence of a crop, nitrate in the soil at the end of a growing season may not be available to the next year""s crop but may serve only to contaminate ground water.
Plants use only those nutrients they need and the use of the nutrients complies with a law of diminishing returns. Above a certain threshold level, the farmer obtains little yield response with increasing nutrient level. From an energy efficiency perspective, nutrients applied above this threshold level are wasted. In the case of a normal distribution with a large coefficient of variation (ratio of standard deviation to the mean value), approximately 50% of the nutrients are wasted. This means that both the energy and raw materials used to manufacture the nutrient, as well as the farmer""s profit dollars, have been squandered.
For example, nitrogen in its gaseous form is of no use to plants. Plants require that nitrogen, in the form of complex nitrogen compounds, be further transformed into soluble nitrates in order to be utilized by the plants. All agricultural chemical compounds, including manure, are toxic to some extent and can contaminate groundwater, particularly those in the nitrate form. Thus amounts of fertilizer greatly in excess of what the plants can profitably use cannot be prudently applied. They are also expensive, which is another good reason to not overfertilize cropland. Until the previous invention, the farmer had no practical way to optimize his application rate, nor to vary his application rate in response to changing conditions across his field. He had been limited to simply applying what worked in the past, perhaps aided by his recollection of how last year""s crop came out, perhaps supplemented by a few spot soil analyses made around the field.
Because of the spatial variations in his field, and because of the time delay between sampling and receiving resultsxe2x80x94during which the soil conditions will have changedxe2x80x94the farmer who has paid for spot samples is scarcely better able to fertilize his fields than is the farmer who simply fertilizes on an historic basis. Consequently, farmers routinely apply excess fertilizer as a protective measure, and in doing so lower their profit margin and risk groundwater contamination, neither of which is desirable.
Farmers know, qualitatively, that crop yields vary because uniformly applied fertilizers are not converted uniformly to forms useful to plants. Farmers generally use rules of thumb to guide application timing. Moreover, farmers realize that their primary source of agrichemical recommendations beyond accepted rules of thumb is either an extension agent or a chemical sales representative.
Soil sampling, used to aid the farmer in fertilizer application, is conventionally based on a farmer""s own sample timing and site selection rationale. Chemical analyses of soil samples that the farmer provides to the extension system agent or salesman require interpretation by technically trained personnel to reveal nutrient needs. Often, however, either no nutrient analysis is performed or the analysis is ignored as meaningless due to the perceived complexity of the technical issues in agricultural chemical management. Today, most generalized nitrogen management recommendations are based on experimental evaluation of different fertilizer treatment methods. Soil tests are not routinely done for available nitrogen at the farm level, and the xe2x80x9cturnaroundxe2x80x9d time between sampling and receiving laboratory results is too long to satisfy the farmers needs for the timing of his application. Local, spatial variations which have significant effects on the crop are normally not addressed at all.
Accordingly, significant energy waste occurred in the application of agricultural chemicals simply because no proven, economical method existed to properly and timely allocate chemicals to meet crop needs, and agricultural chemicals and fertilizers were consequently applied in substantially uniform amounts irrespective of local variations in soil chemical conditions.
In summary, the conventional method of providing agrichemical recommendations for farm level chemical application includes soil sampling by the farmer himself and laboratory analyses, resulting in technically informed interpretations by technically trained personnel. These recommendations normally are then implementedxe2x80x94days or weeks laterxe2x80x94by the farmer himself, who usually is not technically trained in these disciplines.
There are significant sources of error in this multi-step process, including, for example, errors unavoidably caused by the time delay and errors in selecting a truly representative sample, sample collection and handling, sample preparation and conditioning in the laboratory, trained interpretation of nutrient or other chemical needs, and errors in application of the recommended level due to the imprecision of the chemical application equipment.
In a simple form of the invention, and under the proper circumstances, the apparatus and methods of Colburn U.S. Pat. No. 5,033,397 may be used to accurately determine soil deficit or excess conditions in real time, i.e., xe2x80x9con-the-goxe2x80x9d while traversing a field, without the aid of a solvent for such measurements as taught therein. As the calibration factors disclosed therein will rarely remain constant over fields of any significant size,a first improvement to this recent discovery may be effected by intermittently utilizing the solvent distributing apparatus disclosed therein to update such calibration factors on a frequent basis. That is to say, in this, mode of operation the system may operate in a manner opposite to that disclosed in Colburn U.S. Pat. No. 5,033,397; i.e., potential and resistivity determinations may be made in the absence of any solvent, and solvent may be exuded solely for calibration purposes. Although one may store such information for use in the later addition of corrective chemicals, or telemeter such information or the directly measured information to another installation for subsequent use or processing, it is preferred to utilize such information in real time while traversing the sample actually measured to determine and apply the precise amount of agrichemical to the sample measured. In this manner the slight but highly significant positional errors inherent in such bifurcated techniques may be avoided entirely, as may changes in conditions due to the passage of time and the cost and inconvenience of a subsequent field operation.
Beyond the conditions for which the previous invention was intended, or for greater accuracy, it is preferable to measure the complex soil resistivity rather than just the xe2x80x98simplexe2x80x99 soil resistivity (or the teal component of the complex resistivity) as was done previously. It is a further improvement to measure the naturally occurring solute present in the soil, as well as proportional clay content and organic matter, and to utilize the latter to aid in applying chemicals in addition to soil correcting chemicals. In still another improvement, the measuring apparatus may be calibrated intermittently through the aid of either a fluid of two different conductivities or two different fluids of differing conductivities. Such fluid(s) may be of known conductivities or unknown conductivities. Additionally, such calibrating fluids may be xe2x80x9cworkingxe2x80x9d fluids, i.e., the same kinds of fluids being precision-applied to the reference field, such as herbicides and the like.
The system of the Colburn U.S. Pat. No. 5,033,397 invention may be most advantageously employed in applying fertilizer or other corrective chemicals a few weeks after crop emergence. While such post-plant application has been demonstrated to be much more efficient than pre-plant application, it often is necessarily limited to a time duration of Just a few weeks. In many instances it would be desirable to be able to make such determinations without the use of the solvent taught therein, while in other instances it would be desirable to extend the time period during which the localized soil parameters of interest could be determined.
It has been discovered that in adequately moist soils of sufficient warmth, measurement of simple resistivity without the use of solvent remains correlated with available soil nitrate levels sufficiently to permit such measurement without the use of a solvent and a leachate. Preferably, soils being measured without a solvent and leachate will have a moisture content above the wilting point and, for convenience, will be dry enough to permit normal field operations. It is also preferable for the average soil temperature at a few inches depth to exceed approximately 50xc2x0 F. Under these conditions, adequate correlation has been found to permit the efficacious use of the system without an externally applied solvent and resulting leachate.
Beyond these operating conditions, or for greater accuracy within these conditions, it has been found preferable to measure the complex soil resistivity, or impedance. In Colburn U.S. Pat. No. 5,033,397, measurement of resistivity utilizing both time-varying voltages and constant voltages was taught. In the present invention, either constant or time-varying voltages may be used as well, although when measuring complex resistivity it is preferable to utilize time-varying voltages. Also, when employing alternating current excitation, peak voltages are preferably held below about 1.2 volts in order to avoid electrode reaction complications resulting from electrochemical effects at electrode interfaces when measuring complex in situ soil resistivity. If a time-varying voltage is impressed across an electrode-soil system, the propagation of current by direct exchange of electrons is influenced by the complex resistivity or impedance of the system. Complex soil resistivity+ may be considered, by analogy to electric current analysis, as comprised of a real, resistive component (or xe2x80x98simple resistivityxe2x80x99 as used heretofore) and an imaginary, capacitive component of the impedance. If the voltage levels are kept below about 1.2 volts, the charging of the double layer of ions surrounding the electrodes in response to an applied alternating voltage will not produce the oxidation-reduction reactions of U.S. Pat. No. 5,033,397.
Thus complex resistivity includes as subclasses both the electrochemical slurry signal enhancements described in U.S. Pat. No. 5,033,397 at voltage levels appropriate to that subclass and the baseline resistivity signals at lower applied voltages resulting from in situ and solution imbrued soil electrical transmission.
In the present invention, a time-varying voltage regime is preferred since it may provide the most information from interrogation of in situ electrode-soil systems. In a non-steady state electric field, the current I passing through a conductive soil with capacitive reactance is expressed as the ratio of the instantaneous voltage V to the complex resistivity or impedance Ze, or, in equation form, as:
I=V/Ze
The complex resistivity or impedance Ze is itself expressed as:
Ze=[r2+(2Πf C)xe2x88x922 ]0.5xe2x80x83xe2x80x83[1]
where,
C=soil extrinsic capacitance=xcex5A/L
xcex5=soil intrinsic permittivity
r=soil resistivity=L/Ac and
c=soil extrinsic conductance
A=soil area for current flow
L=soil length for current flow
xcex5r=xcex5/xcex5o, the dielectric constant of the soil, and
xcex5o=vacuum permittivity
Soil pore fluid provides both a resistive (real) and a dielectric (imaginary) capacitive medium amenable to transient analysis methods, such as induced polarization or time domain reflectometry which permit soil conductance and dielectric constant values to be determined simultaneously. Alternatively, two or more widely separated excitation frequencies such as 1 KHz and 1 MHz or more can be used to determine the frequency dependence of the complex resistivity.
The benefit of using either of such methods is that the purely conductive component of impedance may be separated from the capacitive component by various circuitry and computational means familiar to those skilled in the electrical arts. The capacitive component has been determined to be functionally related to soil moisture content which is also diagnostically indicative of soil clay content and texture. The conductance component, along with knowledge of soil texture and particle conductance contribution, through heuristic rules or by the further methods herein disclosed, provides the necessary means for determination of representative soil solute levels. For example, if two excitation frequencies are used, the real component of the complex resistivity is not affected by a change in excitation frequency. Thus, two measurements of impedance at two different frequencies provide simultaneous linear equations from which both the resistive and capacitive properties can be extracted. Thus the dual measurements provide the measure of both conductivity (ECa) and water content ("THgr"w).
Recent works by J. D. Rhoades and others provides new formulations for interpreting the temperature-corrected real component of complex soil resistivity for analyses of soil solute levels. These models demonstrate the significance of soil structure and textural base on in situ conductance. The principal conductivity equation of interest to the present invention is given by the relation:
ECa=ECs+T"THgr"wECwxe2x80x83xe2x80x83[2]
where
ECa=in situ measured conductivity
ECs=soil Darticle conductivity
ECw=soil solution conductivity
"THgr"w=soil water content (% wt. basis) and
T=Transmission coefficient
Utilizing these models for determination of soil nitrate levels, for example, requires gain (T) and offset (ECs) factors, each of which is determined by the textural characteristics of the soil and its mineralogy. Thus, these models can be efficiently used by external input of the factors or through independent sensory determination of soil texture such as by the xe2x80x9cfeelxe2x80x9d method and soil particle conductance. In the preferred embodiment, calibration factors may be derived from one or several sources. For example, the relative influence of soil mineralogy on offset factors can be derived from surveys of local clay mineralogy. Such data provide information on relative changes in ground conductivity values that are dominantly influenced by conductive liquid imbibed conductivity characteristics of indigenous imbibed clay minerals, such as kaolinite, xe2x88x92illite, montmorillonite, etc. Such regional characteristics may be input to the controller by the equipment operator or field service representatives. Alternatively, values can be precisely determined by direct comparative measurements of in situ solution imbrued soils. In this method, the soil pores are imbrued with solutions of two different conductivities. Two conductivity measurements respectively provide two simultaneous linear equations which can be solved for the values ECs and T"THgr"w. From, for example, the measurements of complex resistivity, as described previously, a value of "THgr"w may be derived, allowing T to be determined by mathematical substitutions.
Thus, each parameter of interest can be derived from the combination of the equations of the components of the resistivity and the calibration methods described. Substituting these individual parameters reveals the in situ soil solution conductivity (ECw). Since we have observed that soil solutions of midwest agricultural soils are dominantly composed of calcium and soluble nitrate ions (e.g. Ca(NO3)2), soil nitrate levels can be estimated from the measured conductivity relation for pure Ca(NO3)2) solutions. Alternatively, empirical relations obtained from extracts may be used, such as 10 meq/l of NO3xe2x88x92 produces an ECw value of 1000 micro S/cm.
It has been determined that soil particle conductivity is influenced by a soil""s cation exchange capacity. The principal limitation on determining correlations with cation exchange capacity is that absolute values are difficult to assess since each laboratory""s particular methods (e.g. soil grinding and extraction) influence the absolute value of the cation exchange capacity determined. On a regional basis, in a particular soil mineralogy and cropping history, cation exchange capacity has been confirmed to be an appropriately correlated parameter in the major agricultural area of the United States, the Corn Belt.
Cation exchange capacity arises from the negatively charged particle structure of soil colloidal clay minerals and soil colloidal organic matter. In general, the contribution of soil colloidal organic matter total cation exchange capacity dominates over the contribution of clay minerals in the surface soil layer (0-12xe2x80x3) or the xe2x80x9cAxe2x80x9d (or plow depth) horizon. Indigenous clay minerals provide the dominant conductance influence at deeper horizons.
By using the methods of the present invention, the various components of in situ soil complex resistivity can be determined and related to soil parameters of interest for precision application. For example, in the mineral clays of central Illinois, in a silty clay loam with a water content of 20%, an ECs conductivity value of 0.40 mmho/cm would represent a CEC (cation exchange capacity) (Brookside Laboratories) of 25 meq/1 and an organic matter level of 4.4%.
By accounting for the soil particle cation conduction through calibration, the major calibration concern of additive conductive bias (offset) can be eliminated from the local complex resistivity measurement, leaving only the multiplicative bias (gain) of variation in water content for further calibration correction. Multiplicative bias in soil conductivity interpretation accrues through imprecision in assessment of the transmission coefficient. By measuring resistivity at sidedress or at planting time when soil moisture content is representative, multiplicative bias is minimized. Complex resistivity analyses can further reduce multiplicative bias, although with a quantitative determination of offset, empirical or heuristic regional relationships can be employed to determine the gain factors.
Complex resistivity methods of the present invention exploit both macroscopic and local interrogation of soils. In the methods herein described, the steady-state complex resistivity value may be determined by the following relation:
                    ρ        =                              4            ⁢            π            ⁢                          xe2x80x83                        ⁢            aR                    n                                    [        3        ]            
where
xcfx81=soil resistivity (ohm-meters)
a=spacing between measuring electrodes
R=resistance
n=an empirical geometric factor
Thus, although the value of the soil resistivity parameter is independent of the measurement methodology, the geometry of the system determines the resistance value to be measured in the sensing circuitry.
Local measurements between electrodes on the same ground-engaging tool (xe2x80x9caxe2x80x9d measured in centimeters) necessarily create much higher resistance (R) values than do macroscopic measurements between tools on adjacent crop rows (xe2x80x9caxe2x80x9d measured in meters). R is approximately two orders of magnitude lower in macroscopic measurements than in local measurements.
In the case of interpretation of nitrate levels, it is generally found that interrogation of the first foot of soil depth produces a value representative of the upper three feet and measures approximately 50% of the nutrient supplying capacity of the soil. Deeper interrogations provide means of locally interpreting the contributions, particularly for soil regions in which substantial conventional sampling has not been conducted.
Further, in some farming choices, fertilizers are band applied, such as anhydrous ammonia, starter, and manure. Separated electrode arrays offer the advantage that the contribution of bands to crop fertility can be measured by volume integration, rather than discrete sampling.
In the present invention, local resistivity measurements are preferably used in conjunction with the application of known conductivity imbruing soil solutions for calibration, and macroscopic multiple electrode methods are preferably utilized to provide locally averaged values of soil parameters determined from complex resistivity measurements. In the preferred embodiment, calibration of the improved soil resistivity sensor for soil particle conduction, e.g., from organic matter and clay cations, may be effected by linear extrapolation from xe2x80x98two-fluidxe2x80x99 resistivity measurements to the intercept.
It has been determined from agronomic studies of crop response to soil variables that both crop yield and quality are related to spatial variations of soil texture and chemical constituents. The spatial variation of soil clay content in a field influence crop uptake of nitrates, thus influencing crop quality. Further, soil type and textural characteristics have long been used by the USDA Extension system to rate the yield potential of soils. Existing regional recommendations of existing agricultural services may thus be used to assist in fertilizer application on the basis of a component of complex soil resistivity (e.g., ECs or T). Thus crop quality and production may be spatially influenced by treating a soil in response to those soil variables which are revealed through complex resistivety and solution calibration analyses. Multiple agrichemical treatments can simultaneously be prescribed from the sensory measurements of the system and method of the present invention.
Further, it has been observed that the primary limiting nutrients in most cropping situations are the Ca++ and NO3xe2x88x92 ions. High NO3xe2x80x94N levels are favorable to the extraction of calcium from the exchange complex of the soil. This is confirmed by the dominance of the calcium cation and the nitrate anion in soil solutions in the major agricultural region of the United States. Thus through ECa, the contributions of ECs and ECw are combined into a fertility index of the major nutrients influencing crop yield.